As the cost of energy continues to soar, increasing interest is directed toward the development of new sources of fuels. The continuing and ever increasing consumption of fossil resources is of particular concern due both to the consequences of increased global demand for dwindling reserves of easily obtained petroleum oil and the continuing and growing threat of global warming. In particular, the amount of petroleum oil refined and burned as gasoline in order to fuel automobiles in this country and the amount of natural gas, coal and petroleum fuel for central electric power-generating stations continues to increase with no end in sight. An alternative fuel for either or both of these applications is especially desired in view of the amounts of resources consumed and the amount of greenhouse gases generated annually as a result of converting these fuels into energy through combustion.
One possible and very attractive alternative fuel is hydrogen since it produces only water vapor as a byproduct when burned. However, storage of hydrogen for automotive applications is problematic. Storage of hydrogen as a metal hydride has been extensively investigated for at least the last 40 years. Unfortunately, because of thermodynamic and kinetic constraints, the essential properties needed for a hydride storage material (high hydrogen capacity, low reaction enthalpy, reversibility and low desorption temperature) are very difficult to satisfy simultaneously.
Simple binary hydride compounds, such magnesium hydride (MgH2), have been shown promise in that it exhibits good hydrogen reversibility, fast reaction kinetics, and a relatively high hydrogen capacity (7.6 wt %). Unfortunately, MgH2 reaches a hydrogen equilibrium pressure of 1 bar at a temperature of 300° C.: a temperature well above what is believed to be an operating temperature upper limit of about 120° C. for automobile applications.
In order to overcome this shortcoming, several complex metal hydride compounds have been investigated including materials known as alanates. Moreover, borohydride compounds, particularly calcium borohydride, are being investigated for their utility to reversibly store and release hydrogen on demand. Unfortunately, reasonably pure calcium borohydride is not easily prepared. Several of the accepted prior art methods include using diborane (B2H6) gas (see U.S. Pat. Nos. 2,545,633 and 3,224,832), a potentially dangerous material due to its ability to spontaneously ignite in moist air, and as a causative agent for respiratory distress. Current methods appear to utilize sodium borohydride and calcium chloride as precursor materials (L. V. Titov; “Synthesis of calcium borohydride,” Zhurnal Neorganicheskoi Khimii, 1969, v. 13(7): pp. 1797-1800)